2 Wisconsin Academy of Sciences, Arts, and Letters. 
the results obtained from a histological study of the early stages 
of crown-rot. $ 
The purpose of this paper, then, is to summarize in some de¬ 
tail most of the important hypotheses and investigations dealing 
with the matter included in the title, to compare them with one 
another and to bring out their relation to the writer’s observa¬ 
tions. Thus collecting the widely scattered ideas and summariz¬ 
ing the records of research along this line, it is hoped will stim¬ 
ulate a wider interest in the causes of periodic growth in trees 
and encourage and lead to their reconsideration from a more 
modern or quantitative standpoint. In the main the aim is to 
restate the questions raised by the investigators, although some¬ 
times in a modified form. A restudy of the structural and 
tension changes accompanying periodic growth may also lead to 
an investigation of the enzymes active during radial growth and 
to the effect which adverse changes of environment have 
upon them while in an active condition. In any case studies of 
this type will throw more light on the relation of a varying en¬ 
vironment to vegetative and reproductive processes in woody 
plants and thereby increase the knowledge necessary for a com¬ 
prehensive investigation of their diseases. Most of the diseases 
of trees which are of much economic importance and of most 
scientific interest begin in the bark, and their origin seems to 
have a definite relation to such radial growth and consequent 
bark tensions, the normal adjustment of which is interfered with 
by subsequent changes in the environment. Studies of that 
kind will also help to clarify and perhaps correct some misappre¬ 
hension that may exist regarding the relation of mycology and 
physiology to plant pathology. 
Seasonal Periodicity of Growth. 
It is generally held that seasonal periodicity or the alterna¬ 
tion of one or more growing and resting periods during the year 
is a more or less unalterable inheritance of perennial plants of 
temperate zones, but Klebs starting with his extensive investi¬ 
gations on the artificial control of periodicity in algae and fungi, 2 
has reached a very different conclusion regarding the periodic 
3 Klebs, G. Willkiirliche Entwickelungsanderungen be Pflanzen. pp. 166. 
Jena, 1903. 
